1. Field of the Invention
The present invention generally relates to an image forming apparatus, and specifically to an image density control method for the image forming apparatus.
2. Description of the Related Art In recent years, high image quality has been required for image forming apparatuses such as copiers and laser printers. High durability and high stability of images have been also desired. In other words, the images that are formed thereby have to be stably provided at any time even upon the passage of time by minimizing changes in the image quality due to changes in the use environment including continuous printing and intermittent printing.
Has been known a two-component developing method for carrying a two-component developer (hereinafter, “developer”) containing non-magnetic toner (hereinafter, “toner”) and magnetic carrier (hereinafter, “carrier”) on a developer carrying element (hereinafter, “developing sleeve”), which causes a magnetic pole provided therein to form a magnetic brush, and for applying a developing bias to the developing sleeve at a position where a latent image carrier (hereinafter, “photoconductor”) faces the developing sleeve, so as to perform development. The two-component developing method is now widely used because of easy colorization.
In this method, the developer is conveyed to a developing region following a rotation of the developing sleeve. A large number of carrier in the developer aggregate with toner along a magnetic line of a developing pole as the developer is conveyed to the developing region, to form the magnetic brush. The two-component developing method is different from a one-component developing method. In other words, to improve the stability, it is extremely important for the two-component developing method to precisely control a weight ratio, or toner density, between the toner and the carrier.
For example, if the toner density is too high, smudges may appear on the background of an image or a resolution may be reduced. If the toner density is low, the density of a solid image area may be reduced or the carrier may be deposited thereto. Therefore, it is necessary to control the amount of toner supply to adjust the toner density of the developer so as to fall within an appropriate range.
The toner density control is implemented by comparing an output value of a toner-density detector, e.g., permeability sensor, with a control reference value of the toner density to acquire a difference, calculating the amount of toner supply from an calculation equation according to the difference, and supplying toner to a developing device by a toner supply device.
A method of detecting toner density generally uses a permeability sensor. The embodiment of the present invention explained later also employs this sensor. In this method, the change in permeability of a developer due to change in toner density is converted to the change in the toner density.
Another method of detecting toner density uses an optical sensor. This method is implemented by forming a reference patch, which is a pattern, on an image carrier or an intermediate transfer belt and emitting light from a light emitting diode (LED) to the reference patch so as to detect the light that is reflected from the pattern, including specular reflected light or diffuse reflected light, by the optical sensor, e.g., photodiode or phototransistor, and detecting toner density that is amount of toner deposition based on the result of detection.
Has been also known a density control method of forming a reference toner pattern between transfer papers even during printing, during time or distance from end of last image formation to start of current image formation, and of successively controlling a control reference value for toner density of a permeability sensor.
For example, Japanese Patent Application Laid-Open (JP-A) No. S57-136667 and JP-A No. H02-34877 disclose a method of forming a toner pattern in a non-image area, detecting the density of the toner pattern and toner density in a developing device by a unit, and changing a control target value for the toner density in the developing device according to the density of the toner pattern, to maintain image density.
However, many users desire to reduce excessive toner consumption due to actual formation of the toner pattern between sheets of paper as low as possible. Correction based on the formation of the reference toner pattern between the sheets of paper tends to be implemented by increasing an interval of formation of toner patterns, or the toner pattern tends not to be formed. When the toner pattern is formed on the intermediate transfer belt and if a secondary transfer roller is not separated therefrom in each image formation, a toner cleaning device also needs to be provided to clean off the toner of the patch between the sheets adhered to the secondary transfer roller. Furthermore, when the secondary transfer roller is separated in each image formation or in each several image formations, a cleaning device does not need to be provided, but a mechanical mechanism as explained below needs to be provided. The mechanism is required to stand up to contact/separation of the secondary transfer roller with/from the intermediate transfer belt, which frequently occurs.
It is also needed to avoid forming the toner patterns between the sheets of paper in terms of mechanical cost reduction. Japanese Patent No. 3410198 discloses a method of correcting fluctuation in output of a toner density sensor due to change in the state of developer flow according to a period of time for stirring, when toner supply is controlled using the toner density sensor, and of stably maintaining the toner density.
JP-A No. 2001-281979 provides an image forming apparatus that includes a detector for detecting toner density and pattern density, image density, and also includes a unit for further lowering a lower limit when a detected value of the pattern density is a predetermined value or more even if the lower limit is set in a control reference value. However, even if the toner density is kept to a fixed value but if the developing capability of the developer is not stable, the image density is difficult to maintain only by keeping the sensor output to the fixed value.
A number of recently available image forming apparatuses adopt a technique of reducing stress into a developing apparatus. This technique is thought extremely effective in balancing the reduction in the amount of developer and the increase in life of the developer, which have conflicting purposes and are required for downsizing the developing apparatuses.
For example, in a two-component color image forming apparatus, to improve toner dispersion, additives such as silica (SiO2) and titanium oxide (TiO2) are externally added to a large part of toner surface. But these additives are very sensitive to mechanical stress and thermal stress. Therefore, some of the additives are sometimes buried inside the toner or depart from the surface thereof during stirring in the developing device. These phenomena change the flow property and the chargeability of the developer of which a developer including toner and carrier has, and further change physical adhesion between the toner and carrier to change, but the technique of reducing the stress can suppress these phenomena as much as possible.
On the other hand, the reduction of the stress in the developing device may sometimes cause toner chargeability, which is the capability of the developing device to charge toner, to decrease. This phenomenon is explained below. For example, when an image with a low image area ratio, which is small amount of toner replacement per unit time or per unit number of copies, is output, developing capability, that is slope of a graph in which a developing amount of toner is plotted with respect to developing bias, is kept constant. Conversely, when an image with a high image area ratio, which is large amount of toner replacement per unit time or per unit number of copies, is output, the developing capability increases. In other words, a difference in the developing capabilities is caused depending on how much of the toner is replaced in the developer. Because the replacement of the toner causes the difference in the developing capability even if the toner density is the same as each other, the control reference value for toner density has to be changed to keep constant the developing capability upon the passage of time.
Based on the consideration of the properties of the toner, if the former art of controlling is not used, that is, if the complex control, which is performed by the photosensor and the permeability sensor to correct a control reference value for image density by forming toner patches between sheets of paper, is not used, the toner density control by the permeability sensor alone is required to be more accurately performed upon continuous printing and upon changing image mode. Therefore, it is necessary to adopt a method of image density control instead of the former techniques of a complex control.